This application relates to non-invasive, optical probing of various substances, including but not limited to, skins, body tissues and organs of humans and animals.
Investigation of substances by non-invasive and optical means has been the object of many studies as inhomogeneity of light-matter interactions in substances can reveal their structural, compositional, physiological and biological information. Various devices and techniques based on optical coherence domain reflectometry (OCDR) may be used for non-invasive optical probing of various substances, including but not limited to skins, body tissues and organs of humans and animals, to provide tomographic measurements of these substances.
In many OCDR systems, the light from a light source is split into a sampling beam and a reference beam which propagate in two separate optical paths, respectively. The light source may be partially coherent source. The sampling beam is directed along its own optical path to impinge on the substances under study, or sample, while the reference beam is directed in a separate path towards a reference surface. The beams reflected from the sample and from the reference surface are then brought to overlap with each other to optically interfere. Because of the wavelength-dependent phase delay the interference results in no observable interference fringes unless the two optical path lengths of the sampling and reference beams are very similar. This provides a physical mechanism for ranging. A beam splitter may be used to split the light from the light source and to combine the reflected sampling beam and the reflected reference beam for detection at an optical detector. This use of the same device for both splitting and recombining the radiation is essentially based on the well-known Michelson interferometer. The discoveries and the theories of the interference of partially coherent light are summarized by Born and Wolf in “Principles of Optics”, Pergamon Press (1980).
Low-coherence light in free-space Michelson interferometers were utilized for measurement purposes. Optical interferometers based on fiber-optic components were used in various instruments that use low-coherence light as means of characterizing substances. Various embodiments of the fiber-optic OCDR exist such as devices disclosed by Sorin et al in U.S. Pat. No. 5,202,745, by Marcus et al in U.S. Pat. No. 5,659,392, by Mandella et al in U.S. Pat. No. 6,252,666, and by Tearney et al in U.S. Pat. No. 6,421,164. The application of OCDR in medical diagnoses in certain optical configurations has come to known as “optical coherence tomography” (OCT).
FIG. 1 illustrates a typical optical layout used in many fiber-optic OCDR systems described in U.S. Pat. No. 6,421,164 and other publications. A fiber splitter is engaged to two optical fibers that respectively guide the sampling and reference beams in a Michelson configuration. Common to many of these and other implementations, the optical radiation from the low-coherence source is first physically separated into two separate beams where the sampling beam travels in a sample waveguide to interact with the sample while the reference beam travels in a reference waveguide. The fiber splitter than combines the reflected radiation from the sample and the reference light from the reference waveguide to cause interference.